"He who marches out of step hears another drum." ~ Ken Kesey
At least 1", preferably 1.25" or larger. It will run on less but the pipe acts as a reserve tank for surges.Yes on the valve and don't forget the dirt stub.
A plumber can run the gas line if needed. If you have a new gas hookup, the gas company will probably require a pressure test.Check your WC(water column coming in), our oven uses 6" and we have just enough for that and a couple furnaces. There are a total of 5 furnaces in the building and I don't think we could run everything at the same time.